In a nuclear reactor, the fissionable nuclear fuel is, most frequently, in the form of a plurality of individual rods assembled into a bundle of substantially square cross-sections, in such a manner that the rods are held in fixed, spaced relationship. Over a prolonged period of operation of the reactor, the fissionable fuel becomes depleted to the point where it no longer is capable of maintaining or fueling a fission reaction. When this state is reached, it is necessary to remove the rod assembly and replace it with a fresh one. The depleted rod assembly is still of potential value, however, since the rods are still highly radioactive and can be reprocessed in a suitable facility to become capable of sustaining or fueling a fission reaction.
Inasmuch as reprocessing facilities are, more often than not, far removed from the nuclear reactor, it is necessary to ship the spent fuel over long distances, in as safe a manner as possible, both to the outside world and to the rod assembly itself. In order to insure the extreme degree of safety required, the rod assemble is generally loaded into a fuel basket which, in turn, is contained in a shipping cask. It is imperative that the basket and cask assembly be so constructed that harmful radiation does not escape, that the heat generated by the radioactive decay of the spent fuel is adequately dissipated, and that radioactive interaction between the fuel cells is kept below a critical level.
To achieve these ends, numerous types of fuel cells shipping containers have been designed and used, examples of which are disclosed in U.S. Pat. Nos. 4,292,528 of Shaffer et al, 4,543,488 of Diem, 3,962,587 of Dufrance et al, and 4,399,366 of Bucholz.
The Schaffer et al patent discloses a cask for radioactive material in which a plurality of internal fuel containing compartments are formed by a modular construction of surrounding heat conducting members and joined together as by welding, brazing, cementing, or mechanical interfitting. Neutron absorbing material is incorporated into the structure to suppress interaction between the fuel in adjacent compartments. Thus, Shaffer et al achieve the ends of heat dissipation and interaction suppression, as well as radiation suppression through the use of, for example lead shielding.
Diem discloses a basket in which the individual fuel containing tubes are embedded, along with neutron absorbing plates, in a casting of high heat conductivity material, thereby creating an essentially solid, unitary structure having fuel containing tubes extending longitudinally therethrough.
Dufrance et al disclose a basket and cask arrangement in which the basket is suspended within the surrounding cask by a plurality of metallic septa which are for coolant containing chambers. The septa are bonded to the basket and the outer shell to hold the basket firmly in its central orientation.
Bucholz discloses a honeycomb-type structure for the fuel basket which defines a plurality of parallel tubes or cavities for holding the fuel cells. Neutron absorbing material is embedded within the walls of the honeycomb structure in the form of tubes, which may be filled with water to trap neutrons. The walls themselves function as heat conductors to the outer or cask wall.
In all of the foregoing, the aims of suppressed interaction, heat dissipation, and radiation suppression are achieved. However, in these structures, as well as in much of the prior art, the problem of sudden dynamic shock and load in the event of an accident during handling or transportation is not addressed. The dynamic stresses imposed on the fuel basket in the event of an accident, such as, for example, a thirty foot fall by the cask onto an unyielding surface, can be and often are, catastrophic. Structural analysis of state-of-the-art type baskets under impact loading has shown that such baskets tend to suffer greatest stresses at points removed from the point of impact, and that multiple failures of the fuel containing tubes can occur. Further, such analysis has shown that in a substantialy unitary structure as shown in the Diem and Bucholz patents, there can be a failure or rupture of a plurality of fuel containing tubes or cells. A modular structure such as the Shaffer et al arrangement is also susceptible to catastrophic failure, since the tubes are actually formed of a plurality of pieces attached to each other, the points of attachment representing low stress resistance. In like manner, the septa of Dufrance et al are susceptible to detachment from the inner wall of the cask and from the wall of the basket under sudden heavy stress.